I dont remember which tasks. Refreshing didn't helped, but after some time when I completed a task I got 2 new tasks instead of 1, and that goes until I got all 6 tasks back.
Relogin also doesn't help (I tried it the first time it happened)
And I dont know if tasks would add on their own after some time, I waited around a hour first time that happened and nothing changed.
EDIT: I think it has something to do with my connection, it sometimes just stop working for 15-30 seconds.
This just happened to me as well, seems the bug is still happening. I had three at first and now have five after completing some of them, so it does keep adding new ones but I haven't gotten back up to normal yet. I haven't had any issues with connection. I went to bed to see if it fixed itself and it did not, it seems to only start fixing itself little by little when you complete activities. Was kind of shocked when I saw I was "running out" of activities!
Edit: I tried clearing my cache and logging in and out again as well. The tasks I completed were a cooking one (Cook 3), crafting, and I think one other which might have been a 10 activities one, not sure though